Optical mems scanning micro-mirror with anti-speckle cover

ABSTRACT

Optical MEMS scanning micro-mirror comprising:—a movable scanning micro-mirror ( 101 ), being pivotally connected to a MEMS body ( 102 ) substantially surrounding the lateral sides of the micro-mirror,—a transparent window ( 202 ) substantially covering the reflection side of the micro-mirror;—wherein a piezo-actuator assembly ( 500 ) and a layer of deformable transparent material ( 501 ) are provided on the outer portion of said window ( 202 );—the piezo-actuator assembly ( 500 ) being arranged at the periphery of the layer of transparent material ( 501 );—said piezo-actuator assembly ( 500 ) and transparent material ( 501 ) cooperating so that when actuated, the piezo-actuator assembly ( 500 ) causes micro-deformation of the transparent material ( 501 ), thereby providing an anti-speckle effect. The invention also provides the corresponding micro-projection system and method for reducing speckle.

FIELD OF THE INVENTION

The present invention relates to an optical MEMS scanning micro-mirrorcomprising

a movable scanning micro-mirror being pivotally connected to a MEMS bodysubstantially surrounding the lateral sides of the micro-mirror, asubstrate covering a back face of said mirror and a transparent windowsubstantially covering the reflection side of the micro-mirror. Thepresent invention also relates to a micro-projection system comprisingsuch a micro-mirror, a corresponding method for reducing speckle and amethod for manufacturing the optical MEMS scanning mirror.

BACKGROUND OF THE INVENTION

Speckle is a phenomenon created with laser light sources, due to thefact that laser light is coherent. Parallels and synchronized wavefrontssimultaneously hit the projection surface. When the light hits thesurface, it creates constructive and destructive interference. The firstcategory of interference induces an image deterioration that is oftenvisible by human eye and/or by sensors. In addition to a loss of imagequality, visual comfort of the viewer may also be affected.

Several techniques are used in order to remove or reduce speckle. Inmany cases, light coherence reduction techniques are used. For instance,the light hitting the projection surface is provided from variousprojection angles. Polarized laser light hitting a depolarized film isalso used. Otherwise, illumination using various laser wavelengths mayalso be used.

Another approach consists in using vibration of the projection surface.The resulting systems are complex, expensive, and involve very specifichardware material.

WO2009/077198 describes an optical system comprising a coherent lightsource and optical elements for directing light from the source to atarget. The optical elements include at least one diffusing elementarranged to reduce a coherence volume of light from the source and avariable optical property element. A control system controls thevariable optical property element such that different speckle patternsare formed over time at the target with a temporal frequency greaterthan a temporal resolution of an illumination sensor or an eye of anobserver so that speckle contrast ratio in the observed illumination isreduced. The variable optical property element may be a deformablemirror with a vibrating thin plate or film. This solution requiresmodifying the projection system in order to integrate additionalcomponents, such as diffusing elements.

WO2007/112259 describes a system and method for reducing or eliminatingspeckle when using a coherent light source. A refracting device,comprising a birefringent material, is positioned such that therefracting device intercepts the coherent light. The refracting devicerotates, thereby causing the ordinary and/or extraordinary beams tomove. The human eye integrates the movement of the beams, reducing oreliminating laser speckle. The refracting device may include one or moreoptical devices formed of a birefringent material. Wave plates, such asa one-half wave plate, may be inserted between optical devices to causespecific patterns to be generated. Multiple optical devices having adifferent orientation of the horizontal component of the optical axismay also be used to generate other patterns. Furthermore, the refractingdevice may include an optical device having multiple sections ofdiffering horizontal components of the optical axis. This solutioninvolves a complex and expensive component, the rotating refractingdevice. Moreover, the integration of such device requires a specificglobal design.

CN101477234 discloses a piezo-driven optical lens, which comprises alens body, rails for providing an axial motion path, a piezoelectricelement for providing a driving force and an elastic element forproviding pre-stress for contacting the piezoelectric element with therails. The lens body is at least provided with a hollow seat body, alens barrel which is capable of moving axially and is positioned in thehollow seat, and a lens group fixed on the lens barrel. Thepiezoelectric element contacts the rails through the elastic element todrive the lens barrel to move linearly along the rails. This solutionprovides an efficient auto-focus system, but is not adapted to reducespeckle.

WO 9918456 describes a lens with variable focus comprising a chamberfilled with a first liquid, a drop of a second liquid being provided ona first surface zone of the chamber wall, wherein the chamber wall ismade of an insulating material. The first liquid is conductive, thesecond liquid insulating. The first and second liquid are immiscible,with different optical indices and substantially of the same density.Means are provided for positioning said drop in inoperative position onsaid zone, comprising electrical means for applying a voltage stressbetween the conductive liquid and an electrode arranged on said wallsecond surface, and centering means for maintaining the centering andcontrolling the shape of the drop edge while a voltage is being appliedby electrowetting. This solution involves complex liquid/oilencapsulated system and electrostatic actuation.

US2009040602 describes a stress-induced polarization converter in theform of a zero power optical window or, alternatively, a single element,positive or negative power optical lens, that is subject to a controlledamount of symmetric, peripheral stress. The stress may be provided byappropriate mechanical, thermal, hydraulic, electromagnetic/piezo,annealing/molding, or other known techniques. The applied symmetricstress will advantageously be trigonal or four-fold, but is not solimited. This solution involves symmetrical stress, which is not suitedfor speckle reduction.

Thus, there is a need for a novel micro-projection system with reducedspeckle having MEMS micro-mirrors and MEMS components in general, thatdoes not present the above mentioned drawbacks, namely the complexityand costs problems caused by using specific configurations withadditional components used only for speckle reduction.

SUMMARY OF THE INVENTION

A general aim of the invention is therefore to provide an improveddevice and method for reducing or suppressing speckle in a lasermicro-projection system.

Still another aim of the invention is to provide such method and devicefor reducing or suppressing speckle, providing efficient performances atreasonable cost.

Yet another aim of the invention is to provide such method and devicefor reducing or suppressing speckle, using components that can be fullyintegrated into a laser micro-projection device.

These aims are achieved thanks to the optical MEMS scanning micro-mirrorand to the micro-projection system defined in the claims.

There is accordingly provided an optical MEMS scanning micro-mirrorcomprising:

a movable scanning 1D or 2D micro-mirror, being pivotally connected to aMEMS body for deflecting a light beam;

a layer of deformable transparent material arranged to be traversed bysaid light beam;

a piezo-actuator assembly cooperating with said transparent material sothat when actuated, the piezo-actuator assembly causes micro-deformationof the transparent material so as to reduce speckle.

The anti-speckle effect is obtained by arranging and actuating thepiezo-actuator so as to create deformations of the transparent materialwith a short spatial wavelength (preferably much shorter than the outerdimension of the transparent material, thus creating waves) and/or witha high temporal frequency, so as to change the deformation between eachsuccessive frame.

In an advantageous embodiment, the MEMS scanning mirror furthercomprises a package with a transparent window substantially above areflection side of the micro-mirror, said layer of deformabletransparent being unitary with said window.

In a further embodiment, the MEMS scanning mirror further comprises asubstrate under a back face of said mirror;

wherein said piezo-actuator assembly and said layer of deformabletransparent material are provided on the outer portion of said window;

wherein said piezo-actuator assembly is arranged at the periphery of thelayer of transparent material.

In an advantageous embodiment, the piezo-actuator assembly issubstantially circumferential.

In a variant, the piezo-actuator assembly is provided with a pluralityof piezo-elements circumferentially arranged around the transparentmaterial.

The window is preferably provided with a substantially flat outer faceon which the piezo-actuator assembly and the transparent material areattached.

The piezo actuator assembly is advantageously driven on an irregularbasis.

In a preferred embodiment, the piezo-actuator assembly, the transparentlayer and window are preferably fabricated at wafer-level.

The invention also provides a micro-projection system for projectinglight on a projection surface comprising:

at least one coherent light source and preferably a plurality of lightsources with a beam combiner;

optical elements, in the optical path between said coherent light sourceand said projection surface,

an optical MEMS scanning micro-mirror as previously described.

Such a micro-projection system may comprise in addition to themicro-mirror and the light source(s), a quarter-wave plate, a beamsplitter, beam combiner, etc.

The invention further provides a method for reducing speckle in amicro-projection system adapted for projecting light on a projectionsurface, comprising:

providing a light with at least one coherent light source;

directing light from the light source to the projection surface;

providing a scanning micro-mirror for deviating light from said lightsource so as to scan a projected image onto said projecting surface;

providing a layer of deformable transparent material arranged to betraversed by said light beam;

providing a piezo-actuator assembly cooperating with said transparentmaterial so that when actuated, the piezo-actuator assembly causesmicro-deformation of the transparent material so as to reduce speckle.

The piezo-actuator assembly and deformable material configurationenables to deform the transparent material to get an irregular surfacevarying according to the crossing position of the beams. Thus, severalparallel light beams crossing the deformed surface are impacteddifferently (spatial deformation) while the deformation of each pointalso varies with time. Therefore, the deformed surface is specificallyadapted to slightly modify the angle between adjacent light beams,resulting in a reduced coherence, and reduced speckle effect.

In an advantageous embodiment, the piezo-actuator assembly is actuatedso as to create deformations of the transparent material with a spatialwavelength shorter than the average outer dimension of the transparentmaterial, thus creating waves in said transparent material.

In a variant, the piezo-actuator assembly is actuated with a temporalfrequency adapted to change the deformation between each successivepixel.

In a further variant, the above two deformation modes are combined, inorder to enhance the performance.

The invention further provides a method for manufacturing an opticalMEMS scanning micro-mirror, comprising:

providing a MEMS micro-mirror for deflecting a light beam;

providing a transparent substrate arranged to be traversed by said lightbeam;

deposing a piezo-actuator assembly on one side of the substrate, saidassembly forming a substantially central cavity;

providing said cavity with a layer of deformable transparent material.

In a preferred embodiment, the substrate is advantageously a glasswindow.

In the above method, the piezo-actuator assembly is preferably obtainedby:

deposing a metallic layer on the surface of the substrate, for formingan electrode;

removing the excess of material to leave a substantially circumferentialportion for receiving the piezo material;

deposing piezo material on said electrode portion;

removing the excess of piezo material to leave a substantiallycircumferential piezo-actuator assembly;

deposing a metallic layer on said piezo-actuator assembly;

providing at least one layer of deformable transparent material withinthe piezo-actuator assembly.

The method advantageously further comprises the step of providing aspacer wafer (for instance with Silicon or glass) on said substrate, onthe side opposite to said piezo-actuator assembly.

In a preferred embodiment, the method still further comprises the stepof attaching the substrate (for instance by anodic bonding, gluing,eutectic bonding, glass frit bonding, etc) to the reflection side of themicro-mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other purposes, features, aspects and advantages ofthe invention will become apparent from the following detaileddescription of embodiments, given by way of illustration and notlimitation with reference to the accompanying drawings, in which:

FIG. 1 describes a movable micro-mirror;

FIG. 2A and FIG. 2B describe respectively a protected micro-mirror andits cross section;

FIG. 3 is a schematic representation of a projection or scanning systemcomprising a light source and a micro-mirror;

FIG. 4 shows an optical MEMS scanning micro-mirror provided with apiezo-actuator assembly in accordance with the invention;

FIG. 5 shows the optical MEMS scanning micro-mirror of FIG. 4 with aschematic representation of the deformable transparent material when thepiezo-actuator assembly is actuated.

FIG. 6 shows the optical MEMS scanning micro-mirror of FIG. 4 with aschematic representation of the deformable transparent material when thepiezo-actuator assembly is actuated, and the resulting light reflectionsand transmission principles;

FIGS. 7a to 7f, 8a to 8f and 9a to 9c are schematic illustrations of animproved manufacturing process for an anti-speckle MEMS scanning mirrorin accordance with the invention;

FIG. 10 shows a process flow chart with certain process steps carriedout in a fabrication process for fabricating the MEMS components of FIG.4.

DETAILED DESCRIPTION OF THE INVENTION

For clarity, as is generally the case in representation of microsystems,the various figures are not drawn to scale.

FIG. 1 presents a typical rectangular MEMS moving micro-mirror 101,anchored to a fix body 102 by two beams 103, and deflected along itscentral axis.

An example of known type packaged MEMS minor is presented in FIG. 2A andFIG. 2B, where the MEMS minor 101 is protected by a package comprisingin this example transparent or semi-transparent surfaces 201 and 202 asthe incoming light can either come from one side or from two sides ofthe minor surfaces.

The package of the encapsulated MEMS micro-mirror comprises a cap partwith an optical window 202 that allows the light to penetrate andreflects on the micro-mirror surface. The cap optical window istypically made of glass such as borosilicate glass (for instanceborofloat) or other type of glass, and has usually a flat surface.Micro-mirror surface can also be coated with reflective material such asgold, aluminum or silver, deposited in thin film, to obtain strong lightreflection in the visible and Infra-Red wavelength. Optionally, the MEMSmicro-mirror chip can also be packaged by a transparent or opaquesubstrate 201 from the other side of the MEMS micro-mirror chip.Ideally, each of the protection substrates made of transparent materialshould be coated on both sides with anti-reflective coating to avoid anyparasitic light reflection.

FIG. 3 presents a micro-projection system 404 where the light beam,coming from the light source 400, is reflected into the opticalprojection system chip 403, resulting in a projection image 402.

An aspect of the invention consists in reducing or suppressing speckleand therefore improves image quality and stability. FIG. 4 illustrates aMEMS scanning micro-mirror provided with a piezo-actuator 500, attachedto the transparent window 202 of the package for the scanning mirror.The piezo-actuator is preferably arranged with a substantially circularconfiguration in order to form a cavity in which a layer of deformabletransparent material 501 is placed. The latter two elements are in closecooperation so that piezo actuation creates a random or irregulardeformation 502 of the transparent material 501, as shown in FIG. 5. Itis to be noted that the deformation shown in FIG. 5 is voluntarilyoversized, for illustrative purpose only. Actuation of thepiezo-actuator causes alternate contractions and extensions of thetransparent material 501, thereby deforming it, as illustrated. Thedeformation reduces the laser coherence of light crossing thetransparent material, which reduces the perceived speckle effect.

In other words, when light beams providing one pixel or multipleadjacent pixels cross the transparent material in portions havingdifferent deformations, speckle may be reduced or suppressed. To providesuch effect, in an embodiment, a high spatial frequency is used togenerate waves 502, as shown in FIGS. 5 and 6. It is even possible inone embodiment to vibrate the piezoelectric element fast enough tochange the deformation of the deformable material during the projectiontime of one single pixel, thus reducing the speckle of each projectedpixel.

In a second embodiment, the various successive deformations are rapidlygenerated, so that deformations successively supported by the samepixels in different frames are different. This requires a substantiallyhigh temporal frequency, for instance at least equal to, and preferablygreater than the frame frequency.

In a variant, both previous techniques are combined to enhance theresults.

FIG. 6 illustrates exaggerated examples of input light and reflectionwhen the piezo-actuator 500 is actuated. Input light 300 is reflected bythe scanning mirror 101 and guided to the projection target in order toform a projected image. The reflected light 301 from the scanning mirrorpasses through the transparent window 202 and the deformed transparentmaterial 501. The deformation of the latter slightly modifies thedirection of the reflected light, as shown with arrows 302, illustratingthe deviated light. The deviation is minimal and temporary, in order notto affect image quality, but sufficient to reduce the speckle effect.Also this pixel deviation can be compensated by software during theimage projection, so as to change the phase of the laser pulse and sendit slightly in advance or slightly delayed, thus changing its position.

In other words, the different light beams cross the deformable materialat different positions involving different orientations of the material,due to the vibrations of the material. The beams are slightly deviated,as shown in FIG. 6. This reduces the constructive and destructiveinterferences when the beams reach the screen.

FIG. 6 also presents examples of input light 300, either from the top ofthe scanning mirror or from the bottom direction. In the latter case, anangled reflector 310 is required in order to deviate the light to thescanning mirror 101. Such reflector may be provided with silicon orglass coated with a metallic reflective layer, in this example directlyunder window 202.

The circular shape of the piezo-actuator 500 may be continuous along theperiphery of the centrally placed deformable material 501, ordiscontinuous, with regular or irregular interruptions along thecircular profile. The transparent material may be a polymer, siliconessuch as Polydimethylsiloxane (PDMS) or a sufficiently viscous plastic,or other material that is capable of a light deformation when actuatedby a piezo-actuator.

The piezo element 500 and the deformable material 501 are preferablymanufactured at wafer level with the transparent window 202 on whichthey are attached, as explained hereafter.

FIGS. 7a to 7f, 8a to 8f and 9a to 9c are schematic illustrations of animproved manufacturing process for the anti-speckle MEMS scanning mirrorof the invention. These Figures are completed with FIG. 10 showing aprocess flow chart with certain process steps carried out in afabrication process for fabricating the MEMS components. First, the coreMEMS processing is shown (steps 2 to 14 of FIG. 10). A known type SOIwafer 600 (FIG. 7a ) comprising two silicon layers 601 and 603 placed oneach side of a silicon oxide layer 602 is used as a base material formanufacturing the scanning mirror (step 2). FIG. 7b depicts the SOIwafer provided with metal deposition 604 such as aluminum or copper(step 4). FIG. 7c shows the metal layer 605 after etching (step 6). FIG.7d presents the silicon 603 etching, for instance dry or wet etching,followed by oxide 602 etch (dry or wet) (steps 8 and 10). FIG. 7e showsthe silicon 601 dry etch to form the micro-mirror 101 portion (step 12).In FIG. 7f , a transparent glass window 201 made for instance withborosilicate glass (such as borofloat) or other type of glass, is bondedto the wafer (step 14). The glass window may be attached to themicro-mirror chip using any techniques, including but not limited togluing, glass frit bonding, anodic bonding, eutectic bonding, molecularbonding, fusion bonding, low temperature direct bonding, soft soldering,metal thermo compression bonding, bonding with reactive multilayer,laser bonding, polymer attach, etc.

FIGS. 8a to 8f show the different steps of the cap processing. FIG. 8ashows the unprocessed substrate, in this case a transparent glass window(step 16). A metal deposition step is performed as shown in FIG. 8b , toadd a thin layer 700 of metal alloy such as aluminum, to form anelectrode 701 (step 18). Excess of metal is removed by etching (wet ordry) as shown in FIG. 8c (step 20). FIG. 8d shows the deposition of thepiezo material (step 22). A material such as lead zirconate titanate(commonly designated “PZT”) or other natural or man-made piezo materialsuch as Aluminum Nitride (AlN) may be used. Etching enables the removalof not-required piezo material, as shown in FIG. 8e , leaving thepiezo-actuator 500 (step 24). The piezo-actuator section is preferablylarger than the underlying electrode 701. The second electrode 702 isprovided on top of the piezo-actuator 500 by metal deposition, as shownin FIG. 8f (step 26).

A spacer wafer 203 such as Si or glass material is afterwards attachedto the glass window, as shown in FIG. 9a (step 28). The spacer may beattached to the micro-mirror chip using any techniques, including butnot limited to gluing, glass frit bonding, anodic bonding, eutecticbonding, molecular bonding, fusion bonding, low temperature directbonding, soft soldering, metal thermo compression bonding, bonding withreactive multilayer, laser bonding, polymer attach, etc.

A similar type of attachment is used to connect the cap wafer stack tothe MEMS mirror stack assembly, as illustrated in FIG. 9b (step 30).FIG. 9c show the addition of the deformable transparent material 501within the cavity defined by the piezo-actuator 500 (step 32). The layeris preferably substantially flat for proper operation. The addition ofthe deformable material 501 can be done using different techniques,including plastic injection, PDMS or PMMA molding.

What is claimed is:
 1. Optical MEMS scanning micro-mirror devicecomprising: a movable scanning micro-mirrors, being pivotally connectedto a MEMS body for deflecting a light beam; a layer of deformabletransparent material arranged to be traversed by said light beam; apiezo-actuator assembly cooperating with said transparent material sothat when actuated, the piezo-actuator assembly causes micro-deformationof the transparent material so as to reduce speckle.
 2. Optical MEMSscanning micro-mirror according to claim 1, further comprising: apackage with a transparent window substantially above a reflection sideof the micro-mirrors, said layer of deformable transparent being unitarywith said window.
 3. Optical MEMS scanning micro-mirror according toclaim 2, further comprising: a substrate under a back face of saidmirror; wherein said piezo-actuator assembly and said layer ofdeformable transparent material are provided on the outer portion ofsaid window; wherein said piezo-actuator assembly is arranged at theperiphery of the layer of transparent material.
 4. Optical MEMS scanningmicro-mirror according to claim 1, wherein the piezo-actuator assemblyis substantially circumferential.
 5. Optical MEMS scanning micro-mirroraccording to claim 1, wherein the piezo-actuator assembly is providedwith a plurality of piezo-elements circumferentially arranged around thetransparent material.
 6. Optical MEMS scanning micro-mirror according toclaim 1, wherein the window is provided with a substantially flat outerface on which the piezo-actuator assembly and the transparent materialare attached.
 7. Optical MEMS scanning micro-mirror according to claim1, wherein the piezo-actuator assembly, the transparent layer and thewindow are fabricated at wafer-level.
 8. A micro-projection system forprojecting light on a projection surface comprising: at least onecoherent light source; optical elements, in the optical path betweensaid coherent light source and said projection surface, an optical MEMSscanning micro-mirror according to claim
 1. 9. A method for reducingspeckle in a micro-projection system adapted for projecting light on aprojection surface, comprising: providing a light with at least onecoherent light source; directing light from the light source to theprojection surface; providing a scanning micro-mirror for deviatinglight from said light source so as to scan a projected image onto saidprojecting surface; providing a layer of deformable transparent materialarranged to be traversed by said light beam; and providing apiezo-actuator assembly cooperating with said transparent material sothat when actuated, the piezo-actuator assembly causes micro-deformationof the transparent material so as to reduce speckle.
 10. A method forreducing speckle in a micro-projection system according to claim 9,wherein the piezo-actuator assembly is actuated so as to createdeformations of the transparent material with a spatial wavelengthshorter than the average outer dimension of the transparent material,thus creating waves in said transparent material.
 11. A method forreducing speckle in a micro-projection system according to claim 9,wherein the piezo-actuator assembly is actuated with a temporalfrequency adapted to change the deformation within one single pixel,between each pixel or between each successive frame.
 12. A method formanufacturing an optical MEMS scanning micro-mirror, comprising:providing a MEMS micro-mirror for deflecting a light beam; providing atransparent substrate arranged to be traversed by said light beam;deposing a piezo-actuator assembly on one side of the substrate, saidassembly forming a substantially central cavity; and providing saidcavity with a layer of deformable transparent material.
 13. A method formanufacturing an optical MEMS scanning micro-mirror according to claim12, wherein said piezo-actuator assembly is obtained by: deposing ametallic layer on the surface of the substrate, for forming anelectrode; removing the excess of material to leave a substantiallycircumferential portion for receiving the piezo material; deposing piezomaterial on said electrode portion; removing the excess of piezomaterial to leave a substantially circumferential piezo-actuatorassembly; deposing a metallic layer on said piezo-actuator assembly; andproviding at least one layer of deformable transparent material withinthe piezo-actuator assembly.
 14. A method for manufacturing an opticalMEMS scanning micro-mirror according to claim 12, further comprising thestep of providing a spacer wafer on said substrate, on the side oppositeto said piezo-actuator assembly.
 15. A method for manufacturing anoptical MEMS scanning micro-mirror according to claim 14, furthercomprising the step of attaching the substrate window to the reflectionside of the micro-mirror.